1. Field of the Invention
This invention relates generally to polymeric foams and, more particularly, this invention relates to semi-organic foams and in situ foaming processes.
2. Description of the Related Art
Cellular polymeric structures, i.e., foams, are well known in the art and have found extensive use as insulating, packaging and construction materials. Such cellular structures have, in the past, been made from a variety of polymers, including polyurethanes, polystyrenes, cellulose esters and polyvinyl chloride.
Such foam structures can be produced in a variety of forms including, for example, slabs, blocks, boards, sheets, molded shapes, sprayed coatings and extruded insulation. They can also be "foamed-in-place" in an existing space. In such in situ processes, the foamable composition is poured into a space or cavity in a liquid or molten form and allowed to foam. In situ foaming processes are, in many instances, preferred because they greatly facilitate and simplify the necessary production and handling procedures and equipment.
Many conventional foaming processes require that the foamable composition be kept in a substantially dry, i.e., water free, condition. (See, for example, U.S. Pat. No. 3,760,047 issued Sept. 18, 1973 to Gaeth et al and U.S. Pat. No. 3,652,521 issued Mar. 28, 1972 to Abend.) Such systems, because of the difficulty involved with moisture exclusion, find limited practical use.
Urethane foams have become the standard material used for refrigerator insulation. Urethane foams are generally prepared by reacting hydroxyl-terminated compounds called polyols (usually of the polyester or polyether family) with an isocyanate. Generally, such foams are produced by co-reacting the isocyanate and the polyol with processing modifiers such as flow additives and catalysts.
Urethane foams are produced by reactions which can be carried out in a single stage or in a sequence of several stages. Thus, the two principal methods of preparing urethane foams are the one-shot and the prepolymer methods.
In the one-shot method, all the ingredients are mixed together simultaneously and the resulting mixture is allowed to foam.
In the prepolymer method, a portion of the polyol is prereacted with a large excess of the isocyanate to yield a prepolymer. The prepolymer is then subsequently mixed with additional polyol, catalysts and other additives to effect the foaming action. Thus, the prepolymer method necessitates the intermediate step of forming the prepolymerized component.
For ease of production and manufacture, foaming processes whereby the foam is prepared in essentially a one-step process wherein the composition components are mixed and reacted are frequently preferred.
A major disadvantage with the commonly used urethane foams is that one of the starting materials for such foams is an isocyanate which is both a toxic and a respiratory sensitizer to man.